1. Field of the Invention
The invention relates to a process for manufacturing refractory ceramic products such as molds and cores for metal casting. The products made by the process of the invention are characterized by an especially high ability to withstand aggressive molten metals.
2. Discussion of the Prior Art
It is known that ceramic molds are made by mixing a finely divided refractory material with a usually siliceous binding agent. By such prior art methods the refractory particles are mixed with the binding agent and applied to a model by immersion, spraying or pouring. Such molds are hardened by drying or by the use of a liquid, solid or gaseous hardener such as ammonia, the liquid binding agent being transformed to a gel and the particles of refractory material being cemented together. When siliceous binding agents are used, a silica gel forms, which is transformed to silicon dioxide in the firing operation that follows.
Molds and cores made by these known methods, however, do not always produce satisfactory results. Particularly in the casting of types of steel having a relatively high carbon content or types having a high chromium content, reactions often occur between the molten metal and the mold, so that the castings have flawed surfaces.
The use of aluminum alcoholates or alcoholates of metals of the fourth sub-group of the Periodic System in the prior art methods used for the production of ceramic molds has hitherto been impossible owing to the great sensitivity of these compounds to moisture. These alcoholates condense partially, as a result of hydrolysis, with the atmospheric moisture to form products of higher molecular weight which can no longer be used as binding agents. The end product of the condensation reaction is always the corresponding oxide.
It has, therefore, become desirable to provide a ceramic molding composition which can readily be formed to a desired shape and hardened, which ceramic composition is relatively insensitive to water. Moreover, it has become desirable to provide a ceramic molding composition which is substantially free of silica or siliceous materials so that the resultant hardened ceramic object is free from surface defects.